The present invention relates generally to wireless communications systems, and more particularly to the calibration of one or more smart antennas for wireless communications to ensure the performance and signal quality of wireless communications systems.
Smart antenna technology can significantly improve performance and economics of wireless communications systems. It can enable PCS, cellular, and wireless local loop operators to gain significant increases in signal quality, capacity, and coverage area. Operators often require different combinations of these advantages at different times. Systems that offer the most flexibility in terms of configuration and upgradeability are often the most cost-effective, long-term solutions.
Smart antennas, also known as adaptive antennas, have a distinct advantage in modern wireless communications systems. A smart antenna is capable of beam forming or directing a beam of radiated energy toward a desired receiver. The dual purpose of a system deploying one or more smart antennas in an antenna array is to augment the signal quality of the radio based system through a more focused transmission of radio signals without reducing signal capacity. One advantage of this capability is to direct more power to the desired receiver. Another advantage of using smart antenna arrays for beam forming is the ability to reduce the transmitted power due to the more directional nature of smart antennas. Finally, a system deploying smart antenna arrays typically improves the channel conditions, such as a signal-to-interference ratio of the received signal, between any smart antenna array and the desired transceiver.
In order to accurately form a desired beam, the amplitude and phase of each component of the adaptive array sub-system should be known with a reasonable degree of precision. Un-compensated differences in gain and phase in a system with smart antennas degrade the antennas' performance. Ideally, the gain and phase characteristics are predetermined at the time of manufacture and are environment-invariant. However, in reality these characteristics vary over time and different environments. Thus, a method and apparatus to calibrate the antenna array is needed. Specifically, a method to determine variations in gain and phase of a system having one or more smart antennas and a method to compensate for those variations are needed.
One conventional calibration method is the “remote subscriber/transponder assisted calibration”. This approach requires the assistance from a remote subscriber/transponder unit with a predetermined location. A set of N orthogonal calibration signals needs to be generated and then transmitted from each antenna to allow the subscriber/transponder to calculate the phase and power of each signal from each antenna, where N is the number of the antennas in an antenna array. Furthermore, the subscriber/transponder shall be placed at a line of sight (LOS) location to the antenna array, otherwise the air channel effects due to multi-path may significantly degrade calibration accuracy.
Another conventional calibration method is the “on-site calibration with a collocated calibration unit”. This approach requires a special collocated calibration unit and involves generating and injecting special calibration signals into the transmitter and receiver chains. The collocated calibration must have the ability to compute the phases and powers of multiple signals (calibration signals).
Both conventional approaches require a special calibration period during which special calibration signals are generated and injected or transmitted to the calibration unit. This causes a disruption to the normal system operation. Moreover, both approaches require a special calibration unit or subscriber unit/transponder that has the capability to detect simultaneously both the phase and power of multiple calibration signals. Both the said disruption and the need for special equipment can be prohibitively costly in certain wireless communications system designs.
Therefore, desirable in the art of smart antenna array designs are improved array calibration systems and methods that ensure the performance and signal quality of wireless communications systems.